Evaporation is an extremely powerful cooling process. When totally evaporated from the skin surface, sweat can remove body heat at a rate of 2.43 kJ«g"\ Humans therefore control sweat secretion to maintain thermal homeostasis. Since humans are capable of extended sweat rates approximating 30 g'min"1, it is possible to remove heat at rates -73 kJ-min"1. Assuming a 20% efficiency, such heat loss will support a normothermic total energy use of 1520W. This equates with an external work rate of 304W, eliciting an oxygen consumption >3.5 /«min"1. However, while man has a great capacity to both work and dissipate metabolically-derived heat, exercise under various environmental extremes may impede heat dissipation. Under such conditions, the cumulative effects of metabolic and environmental thermal loads may represent an uncompensable heat stress, predisposing to hyperthermia.